As a spark plug mounted to an internal combustion engine, a spark plug is known that includes a tubular insulator to which a center electrode is fixed, and a tubular metal shell disposed on the outer circumference of the insulator, and that causes discharge in an inter-electrode spark gap. In this type of spark plug, charges stored by parasitic capacitance between the center electrode and the metal shell flow into the spark gap at the time of discharge, and electrode erosion may be thus caused. In order to reduce the parasitic capacitance and suppress the electrode erosion, International Publication No. 2016/174816 discloses a technique in which a recessed portion (an air layer having a dielectric constant lower than the dielectric constant of an insulator) is provided in the outer circumferential surface of the insulator.
However, in the above-described conventional technique, the recessed portion (air layer) formed in the outer circumferential surface of the insulator suppresses heat transfer from the insulator to a metal shell, and thus, the heat dissipation property is reduced accordingly. As a result, there is a risk that the insulator is overheated and pre-ignition comes to easily occur.
The present invention has been conceived to address the above-described problem. An advantage of the present invention is a spark plug that enables improvement in heat dissipation property.